1. Field of the Invention
The present invention relates to a control device for a hybrid electric vehicle, and more specifically, to a control device for a hybrid electric vehicle capable of transmitting a driving force of an engine and that of an electric motor to driving wheels of a vehicle.
2. Description of the Related Art
A so-called parallel hybrid electric vehicle capable of transmitting the driving force of the engine and that of the electric motor to the driving wheels of a vehicle has conventionally been developed and in practical use.
As a parallel hybrid electric vehicle, a hybrid electric vehicle in which there is provided a clutch that mechanically connects/disconnects the engine and the automatic transmission, and the rotary shaft of the electric motor is coupled to between the output shaft of the clutch and the input shaft of the automatic transmission, is proposed, for example, in Unexamined Japanese Patent Publication No. 5-176405 (hereinafter, referred to as Patent Document 1).
In the hybrid electric vehicle as shown in Patent Document 1, the clutch is disengaged at the start of the vehicle, and the electric motor is operated as a motor by power supply from the battery. Therefore, the vehicle starts traveling by using only the driving force of the electric motor. During the traveling of the vehicle after startup, the clutch is engaged, and the driving force of the engine is transmitted to the driving wheels through the transmission.
At the time of deceleration of the vehicle, the electric motor is operated as a generator to create a regenerative braking force. Regenerative braking energy that is produced in this process is converted to electric power and charged to the battery.
When the accelerator pedal is released, and the hybrid electric vehicle in deceleration with the service brake system of the vehicle not operated, a deceleration torque is determined as a required deceleration torque, which can achieve virtually the same deceleration as the deceleration produced when a similar vehicle whose power source is only an engine is decelerated in a similar manner. The electric motor and the engine are controlled so that the required deceleration torque is obtained.
An upper limit deceleration torque is prescribed in the electric motor, which is a maximum value of a regenerative braking torque that can be generated depending upon the specifications of the electric motor. The electric motor is capable of generating the regenerative braking torque up to the upper limit deceleration torque.
The required deceleration torque and the upper limit deceleration torque of the electric motor, which are used here, have relationship as shown in an upper graph of FIG. 6. That is to say, as the revolution speed of the electric motor is increased, the required deceleration torque becomes large. The upper limit deceleration torque is at a constant value in low revolution. In high revolution, the value of the upper limit deceleration torque becomes smaller as the revolution speed of the electric motor is increased. When the revolution speed of the electric motor is a revolution speed Nx′, the required deceleration torque and the upper limit deceleration torque are equal to each other.
If the revolution speed of the electric motor is higher than the revolution speed Nx′, the required deceleration torque cannot be obtained only from the regenerative braking torque generable by the electric motor. Therefore, the clutch is engaged, and the engine and the electric motor are controlled so that the required deceleration torque may be obtained by combining the deceleration torque of the engine with the regenerative braking torque of the electric motor.
If the revolution speed of the electric motor is equal to or lower than the revolution speed Nx′, it is possible to obtain the regenerative braking torque equal to the required deceleration torque from the electric motor. Therefore, the clutch is disengaged, and the electric motor is controlled so that the required deceleration torque is generated only by the regenerative braking of the electric motor. This allows as much deceleration energy as possible to be converted to electric power and to be returned to the battery.
If an automatic transmission is used in the hybrid electric vehicle thus constructed, when the vehicle is in deceleration, the automatic transmission is gradually downshifted to lower gears along with a reduction of the traveling speed, that is, a reduction of the revolution speed of the electric motor.
The shifts of gears of the automatic transmission and corresponding changes of the revolution speed of the electric motor are shown by a solid line in a lower graph of FIG. 6. Straight chain lines in FIG. 6 indicate relationships between traveling speeds at different gears and the respective revolution speeds of the electric motor with respect to each gear. Hereinafter, these chain lines will be referred to as speed-change lines.
In a state where the downshift of the automatic transmission is not being carried out, as the traveling speed decreases, the revolution speed of the electric motor changes in the decreasing direction along the speed-change line corresponding to the present gear. When downshift is carried out in the deceleration according to a predetermined gear shift map for downshift, the revolution speed of the electric motor increases while moving from the speed-change line corresponding to the gear before the gear shift to the one corresponding to an adjacent lower gear, which is the selected gear after the gear shift.
For instance, if the automatic transmission has five forward gears, and the vehicle is brought into a deceleration running state by releasing the accelerator pedal with the fifth gear selected, the revolution speed of the electric motor changes in the decreasing direction along the speed-change line corresponding to the fifth gear as the traveling speed decreases. At this time, as shown by the solid line that is drawn along the speed-change line corresponding to the fifth gear in FIG. 6, the revolution speed of the electric motor passes through the revolution speed Nx′ at which the required deceleration torque and the upper limit deceleration torque become equal to each other. Consequently, when the revolution speed of the electric motor passes through the revolution speed Nx′, the clutch is switched from an engaged position to a disengaged position.
When the traveling speed decreases to V4′, the automatic transmission is downshifted from the fifth to the fourth gear. Along with this downshift, the revolution speed of the electric motor increases while moving from the speed-change line of the fifth gear to that of the fourth gear as shown in FIG. 6. At this time, the revolution speed of the electric motor passes through the revolution speed Nx′ again. When the revolution speed moves from a state being lower than the revolution speed Nx′ to a state being higher than the revolution speed Nx′, the clutch is switched from the disengaged position to the engaged position.
If the vehicle is continuously decelerated after the downshift to the fourth gear, the revolution speed of the electric motor changes in the decreasing direction along the speed-change line corresponding to the fourth gear as in the case where the gear is the fifth. Again, the revolution speed of the electric motor passes through the revolution speed Nx′, so that the clutch is switched from the engaged position to the disengaged position as in the case of the fifth gear.
When the traveling speed further decreases to reach V3′, the automatic transmission is downshifted from the fourth to the third gear. In response to this downshift, the revolution speed of the electric motor increases while moving from the speed-change line of the fourth gear to that of the third gear as shown in FIG. 6. At this time, the revolution speed of the electric motor passes through the revolution speed Nx′ again, so that the clutch is switched from the disengaged position to the engaged position.
If the vehicle is continuously decelerated after the downshift to the third gear, the revolution speed of the electric motor changes in the decreasing direction along the speed-change line corresponding to the third gear. Again, the revolution speed of the electric motor passes through the revolution speed Nx′, so that the clutch is switched from the engaged position to the disengaged position as in the cases of the fifth and fourth gears.
Thereafter, if the traveling speed reaches V2′ in response to a further reduction of the traveling speed, the automatic transmission is downshifted from the third to the second gear. When the traveling speed reaches V1′, the automatic transmission is downshifted from the second to the first gear. In response to the reduction of the traveling speed and the downshift of the automatic transmission, as in the foregoing cases, the revolution speed of the electric motor decreases although with moving from the speed-change line corresponding to the third gear to the speed-change line corresponding to the second gear and moving from the speed-change line corresponding to the second gear to the speed-change line corresponding to the first gear.
If the traveling speed decreases due to deceleration of the vehicle, the engagement/disengagement of the clutch are repeated over and over every time the gears of the automatic transmission are sequentially downshifted from the fifth gear. The repetition of the engagement/disengagement of the clutch causes the problem that the clutch is increasingly abraded away and is degraded in durability. There is another problem that drive feeling is deteriorated due to an increase in vibrations and noises caused by operations of the clutch.
The clutch engagement requires that engine revolution be adjusted to the revolution speed of the electric motor beforehand for the engagement to be smoothly performed. Since, as described above, the clutch is frequently engaged/disengaged at the time of deceleration, the engine revolution is also changed very often. Consequently, there arises a problem of degradation in fuel economy.